The present invention relates to devices for transmitting light to, and receiving light from, a remote sample to be analyzed, and, more particularly, to optical fiber head probes suitable for multi-purpose applications, such as those involving transmission spectroscopy, light scattering spectroscopy, fluorescence, and index of refraction measurements.
A number of devices which are available for use in the spectral analysis of remote scattering. Various of these devices may be used to measure the reflection, the transmission, the fluorescence or the light scattering from the remote samples.
Such devices are typically made up of three parts. Such systems feature an analyzer, which include a light, or other radiation, source and a detection system. A second component is an optical probe head of an appropriate type, for transmitting the light, or other radiation, to, and receiving it from, a sample. Finally, the systems feature suitable fiber optics, for guiding the light, or other radiation, between the analyzer and the probe head.
The present invention relates to the probe head and specifically to probe head which can be used in conjunction with any of a wide variety of analyzers and fiber optics systems in transmission spectroscopy, Raman spectroscopy, and in index of refraction measurements.
A number of different devices are used for remote sensing. At least one device, described in T. Davidson, D. Tracy, A. Lokshin, K. DeCondre, L. McDermott, The Perkin Elmer Corp. in the Pittsburgh Conference, Atlanta (1993), describes the design of a probe head capable of measuring the absolute transmission signal of the sample. The probe described therein includes dual cells, one of which is for sample while the other is a dummy reference cell. A mechanical shutter is used to alternately block and unblock the sample and reference optical path. Such a probe suffers from a number of disadvantages. First, such a probe is made up of many optical components, such as lenses, a beam splitter, prisms, optical windows, and the like, which make it awkward, expensive and difficult to properly align. Second, the probe is inefficient in that at least 3/4 of the signal is lost in the course of double pass through the beam splinter, used to split the beam to the self-reference and sample optical paths.
A typical optical probe head available on the market is disclosed in U.S. Pat. No. 5,044,755 by I. Landa et al. The probe disclosed therein is designed to measure transmission. In this particular design, the light emerging from a fiber bundle is collimated by a lens. The optical ray is then guided through a sample compartment and is reflected back to the same lens which focuses the light into the same fiber bundle. Some of the fibers are used to guide the light into the probe while some of the fibers are used to guide the light out to the detection system.
Another type of probe, used by UOP Guided Wave Inc. and by Galileo Electro-optics Corp., is a transmission probe in which the light emerging from the fiber, whether a single fiber or a fiber bundle, is collimated by a lens which guides the light through the sample compartment. On emerging from the sample compartment, the ray is collected by another lens which focuses the optical ray onto a second output fiber.
Neither of the two probe types described above carries a self-reference channel for use in correcting the fiber optical response.
U.S. Pat. No. 5,112,127 by M. M. Carrabba, disclosed the design of an optical probe head for measuring Raman scattering. The device described therein is made up of many optical elements, including three lenses, a beam splinter, a filter and a prism. The device is difficult to align and is capable of probing only very small sample volumes, which may be adequate for sampling opaque materials but is undesirable for applications involving transparent liquids.
S. D. Schwab and R. L. McCreery, in Anal. Chem. 56, 2199 (1984), disclose a simple design for a Raman probe having no optical elements. A fiber bundle is used, with the inner fiber serving in the excitation while the outer fibers are used to collect the scattered light. To use the device, the bundle tip is simply immersed into the specimen to be sampled. One disadvantage of this probe is that, because of the large acceptance angle of the fibers, the device picks up room light, which, even at very low light levels, can be much stronger that the Raman signal.
There is thus a widely recognized need for, and it would be highly advantageous to have, a multi-purpose probe for use in remote sampling which will be simple and reliable and which will be easy to align and operate.